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About five years ago, writer and renewable energy aficionado Warren Weismann was researching ancient Greece for his novel when he stumbled across information that the Greeks had built anaerobic digesters to produce methane. He then read about similar archaeological evidence in ancient Syria and China. But it was the modern biogas boom in China that got him most excited and distracted him from his writing career: Tens of millions of home-scale biodigesters have been built in China over the last century, with the pace of construction still accelerating. Warren wanted one for himself.
After a few years of further research, including conversations with colleagues in India and Nepal, where small-scale biogas production is prevalent, Warren modified traditional designs to create a plan for his own 700-gallon biodigester. He was living at Maitreya Ecovillage, a threeblock community and green-building-oriented neighborhood near downtown Eugene, Oregon. After building his first biodigester last year, he’s become increasingly excited about the possibilities for home-scale biogas, and has established Hestia Home Biogas to build biodigesters locally and consult on biodigesters across the globe.
Back from Obscurity
Biogas has been used for lighting for at least a century, and possibly millennia. But it was mostly abandoned in the United States after cheap and abundant fossil fuel was harnessed in the early 20th century. Home-scale biodigesters have remained on the sidelines in the developed world, but are poised for a comeback as interest in a replacement fuel increases.
There are good reasons to consider building biodigesters for a community, small farm, or even home. Biodigesters yield two products that are extremely useful for the home and garden—high-nitrogen compost and flammable gas.
Biodigesters anaerobically (without air) break down organic matter in a slurry held in a tank. The nitrogen remains in the composted slurry as ammonia, a vital plant nutrient. The flammable gas produced by biodigesters is about twothirds methane and one-third carbon dioxide—very similar to natural gas—making it a good cooking fuel. Cooking requires intense direct application of heat on demand, and renewable options for accomplishing this are limited. Solar energy is dispersed and not consistently available, making solar cooking challenging, and burning wood contributes to particulate pollution and further depletes diminishing resources in the developing world. Cooking is not a huge consumer of energy in the industrialized world, but doing it more sustainably is challenging. Unlike cooking with solar electricity, biodigesters can be assembled with readily available materials by a handy homeowner. Any type of propane or natural gas stove will run on biogas. For maximum efficiency, propane stoves will require a larger air inlet.
Biodigester Efficiency and the Environment
Cooking fuel is greatly needed in less industrialized nations, especially in rural areas. Several countries, such as India andCosta Rica, provide crucial government support for biodigester technology, but none more so than China. More than 30 million biodigesters have been built there, supplying renewable cooking and lighting gas for more than 100 million people.
Anaerobic composting and its biogas production have major advantages compared to traditional aerobic composting and burning biomass for cooking and lighting. These advantages are opening the door to a more sustainable rural economy in China, especially in Sichuan province, where the modern biogas movement began and government support and technical know-how is strongest.
Probably the greatest advantage with biodigesters is in their efficiency—biogas often achieves efficiencies of 60%, compared with about 10% for the typical homemade biomass-burning cook stove. Using less biomass means more living trees, less air pollution and greenhouse gas emissions, and a vast improvement in household air quality. Local water quality and sanitation is also greatly improved, as human and animal wastes can be composted in a sanitary manner. And lastly, the end product is a quality fertilizer, rich in nitrogen and free of pathogens. While there is much in China to bemoan on the environmental front, the more than 4 million biodigesters being built in rural China each year certainly provide one of the bright spots in a nation lurching toward industrialization.
Inside a Biodigester
A biodigester is a sophisticated way to harvest fuel from the complex carbon chains of organic matter—energy collected by plants from the sun as they grow—without combusting them directly. Direct combustion of carbon causes air pollution, a loss of much of the nutrient value of the biomass, and a poor energy harvest—especially when used for cooking or lighting, as most of it goes up in smoke. Burning wood, even in an EPA-certified woodstove, can produce more than 500 times the fine particle emissions of burning natural gas.
As new plant material is fed into the digester, it is first attacked by acidogenic bacteria, which break the chains holding together some of the more complex plant matter, especially cellulose—the structural backbone of most plants. Ammonia and acetates (mostly acid) are produced, lowering the pH and using up any oxygen in the process. Acetates are the perfect food for methanogenic bacteria, as long as the slurry they reside in is not too acidic and all oxygen has been removed. They consume the acetates and produce methane (CH4) and carbon dioxide (CO2), along with a lesser amount of other gases and residues depending on the original feedstock.
In consuming these acids, the methanogenic bacteria raise the pH and keep it hospitable for both the acid-formers and themselves, both of which would perish if the pH dropped too low. The high-nitrogen ammonia (a byproduct of the breakdown of plant proteins) remains dissolved in the slurry, unlike in aerobic composting where it is released as a gas. Although both the acid-formers and the methanogens can suffer from rapid changes in living conditions created by the addition of feedstock, the methanogens are especially vulnerable to low pH and the introduction of too much oxygen. For this reason, biodigesters generally work on the principle of steady applications of new feedstock in regular intervals, rather than adding large amounts of biomass at once.
There are several different types of methanogenic bacteria that will colonize a digester, depending upon the slurry’s temperature range. The two ranges of interest to homescale biodigesters are the cryophilic (50°F to 80°F) and the mesophilic (95°F to 125°F). There is a dead zone between these two temperature ranges that must be avoided. Warren’s biodigester operates in the cryophilic range. While mesophilic methanogens can break down material several times as quickly as their cryophilic counterparts, consistently maintaining high temperatures consumes a great deal of energy—which can make a net energy loss for smaller biodigesters.
Building a Biogas Biodigester
Hestia biodigesters are approximately 5 by 7 feet wide by 5 feet deep, providing about 700 gallons of capacity. Slurry occupies about 600 gallons of this biodigester; the remaining space is for the gas that’s produced. The design is straightforward: an insulated concrete vessel is topped with a steel frame that holds an EDPM pond liner, which expands as gas is produced. There’s an inlet for adding feedstock and an outlet for removing composted slurry. A closed loop of PEX tubing in the bottom of the tank is plumbed to an on-demand water heater to add heat when the slurry temperature drops below 50°F—the temperature at which cryophilic methanogenic bacteria go dormant and stop producing gas. If the climate is mild, it may be enough to build a hoop house over the tank to keep the slurry sufficiently warm in winter. Alternatively, the biodigester could be allowed to go dormant during the colder months.
The first step in building the Hestia’s biodigester is to excavate 28 inches below grade, which makes the height of the inlet right for easy addition of feedstock by 5-gallon bucket. Warren likes to make sure the digester is visible from the kitchen, since the inflation of the rubber top indicates if there is sufficient gas available for cooking
Alternatively, a simple pressure gauge could be added to the gas line in the kitchen.
After excavation comes building the wall forms for the 2 cubic yards of aggregate-free concrete, which must be poured all at once to avoid leaks through the walls. A 4-inch PVC outlet pipe and any PEX tubing for adding hydronic heat must be set in place. The PEX tubing will rest on the bottom of the floor of the tank, so two short pieces, one for entry and one for exit, must be embedded in the wall so the rest of the radiant heating system can be attached later.
A concrete truck with a pump is best to fill the forms in one pour. A concrete vibrator (also called a “stinger”) will help remove air bubbles’ weak spots from the concrete. The massive weight of the wet concrete and the agitation of the stinger make it important to solidly secure the forms.
After the concrete cures and the forms are removed, three coats of “moose-milk” finish—a mix of Portland cement and acrylic latex—is painted on to help prevent leaks. The first is a bonding coat of watery-thin consistency. The second coat is thicker (like peanut butter), with a higher ratio of Portland cement. The finish coat is another thin application. After sealing, the tank is filled with water for a leak test. If this goes well, the outside of the tank can be insulated with 3 inches of rigid foam board insulation and then backfilled. For aesthetics, Warren wraps the exterior of the biodigester in chicken wire and then stuccos it.
The 40-mil pond liner and steel frame that serve as the tank top are held in place with 18 anchor bolts inserted into the top of the concrete tank at the time of the pour. The steel frame can be fabricated or purchased from Hestia.
The gas line is attached to a regular barbed fitting secured with a hose clamp. The rubber membrane is sandwiched between two washers and nuts on the threaded end of the fitting. The gas line is a 1/2-inch flex hose, which is transitioned to a standard PVC gas pipe when it goes underground. Burying the PVC line protects it from photodegradation and developing cracks that could lead to gas leaks.
The most common problem is water buildup in the gas line, which can interfere with gas flow. When this occurs, the gas line must be picked up and the water drained back into the digester. Ideally, a water separator could be combined with a pressure-relief valve on the bottom to eliminate excess moisture when the valve is tripped.
The released CO2 and CH4 gases percolate through the slurry to the top of the tank. Once enough biogas accumulates, the pressure created inside the expanding rubber top reaches 0.25 to 0.5 psi, enough to move the gas through the delivery pipes and use it for cooking or lighting. As new material is added to the tank through the inlet, it displaces an equal amount of slurry through the outlet, which can be applied directly to the garden. Once applied, covering the slurry with soil helps keep the ammonia from turning to gas and losing its coveted nitrogen.
Operating the Digester
Warren’s biodigester at Maitreya primarily uses kitchen waste once in operation. Getting it up and running, however, requires a whole mess of ruminant manure—about 300 pounds’ worth. There happened to be an alpaca farm nearby, and so he used alpaca manure, but any ruminant manure will be loaded with plenty of methanogenic bacteria. He also added a few gallons of kitchen compost and tree leaves, and then filled the tank to 600 gallons.
It takes a minimum of two to three days before a biodigester begins to produce gas, since the acid-forming bacteria need to do their work before the methane-producing bacteria can go to work. About 10 to 15 pounds of kitchen scraps are collected and added to the biodigester daily, producing an average of about 70 cubic feet of biogas, which is the only source of cooking fuel for the community’s kitchen. When the methane content begins to get low, the flame will begin to burn orange. This is remedied by feeding the digester.
The scraps are shoved into the digester with a pole, and this slight agitation helps mix the slurry, exposing the material to the bacteria so it can be thoroughly composted. The tank can also be topped off with water at this time if the slurry level is low. As new feedstock is added, it displaces an equal volume of composted slurry through the outlet, which is captured in a 5-gallon bucket and then added to nearby gardens. Biodigesters prefer a carbon-to-nitrogen ratio similar to a conventional aerobic compost pile, with about 25:1 being ideal. Too much carbon-rich material (grass clippings, newspaper, etc.) will slow the digester. Manure is the most common nitrogenous material to add to rebalance a slow digester.
The biodigester at Maitreya has been operating for more than a year with consistent results. Warren and Hestia Biogas are in the final stages of getting city permits for new biodigesters, to bring an official stamp of approval to a renewable energy technology with great promise for any homestead.
Biogas Biodigester Tank Alternatives
Besides building your own biodigester tank, repurposing old food storage or other tanks is a possibility for biogas generation. You’ll just need to size it correctly and make sure it’s leakproof. A general rule is that the tank needs to be 50 times the size of the daily input to allow for some space for gas to collect. If your input is 15 gallons of material per day, you’d need a 750-gallon tank.
Construction Time and Costs
For this particular biodigester, construction time will vary depending on a person’s construction experience or if a professional concrete contractor is hired to pour the digester tank. Time will vary from several weekends to several days, separated by a seven-day concrete-curing period.
Materials will cost $1,000 to $1,200, depending on the price of ready-mix concrete in your area and if the concrete company will charge you a “short-load” fee for ordering only 2 yards of concrete. The project can be broken down into excavation and concrete; plumbing and gas piping; and external masonry. The concrete, rebar, battens, and anchor bolts run about $500. Warren highly recommends spending the extra $200 to $300 for a concrete pumper truck to avoid having to “bucket brigade” the concrete. The plumbing and gas piping will be an additional $200, and external insulation and masonry, another $300. (To purchase a complete plan set for $89, visit www.HestiaHomeBiogas.com.)
Biodigester Maintenance, Costs and Risks
Biodigesters are living things, and just like a vegetable garden or flock of chickens, they require regular maintenance to function properly. It’s important to avoid adding materials that can overwhelm or clog up the digester, including wood or plant stalks thicker than your finger; large amounts of meat; bones (unless they are ground up); fresh citrus or apples; and fresh chicken manure (which is OK only if it’s allowed to dry first). A few 5-gallon buckets of digested slurry will need to be removed every few days for a 700-gallon digester like the one at Maitreya, but otherwise the digester shouldn’t ever need to be cleaned out.
Regular use of the gas is important to avoid explosion hazards. Letting the digester sit for more than a week can create an abundance of biogas. Operating the digester at its natural pressure, without further gas pumps to pressurize it, in addition to the appropriate check- and pressure-relief valves, helps ensure that an unsafe buildup of gas doesn’t occur.
A biodigester of this size should be cleared with your local fire marshal, who may or may not be sympathetic to its construction. Biodigesters are not common in North America, and some education of officials will likely be required. Warren has been working with the City of Eugene for formal approval, and the case is pending.
Author and builder Stephen Hren (StephenHren@gmail.com) lives in Durham, North Carolina. His latest book is Tales from the Sustainable Underground: A Wild Journey with People Who Care More About the Planet Than the Law.
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